Breed, maintain, and monitor selected strains of rats, namely male and female rats of the following inbred strains and hybrid crosses: F344 x F344; BN x BN; BUF x BUF; LEW x LEW; F344 x BN; F344 x BUF; and F344 x LEW. The purpose is to develop a rat model for aging with standardized "random-bred" characteristics.